The present invention relates generally to a steering system for a vehicle and more particularly to a control system for a steer-by-wire steering system.
In a steer-by-wire steering system, the vehicle""s steering wheel is disengaged from the steering mechanism. In such a steering system, there is no mechanical coupling between the steering wheel and the steering mechanism.
Even though the mechanical linkage between the steering wheel and the road wheels has been eliminated, a steer-by-wire steering system is expected not only to produce the same functions and steering feel as a conventional mechanically linked steering system, but it is also expected to implement advanced steering system features. Requirements for conventional steering functions and advanced steering features such as adjustable steering feel can be implemented by an advanced control system design.
A steer-by-wire control system must satisfy high quality steering requirements and functions, such as a vehicle directional control requirement, a steering wheel to road wheel synchronization requirement, adjustable steering effort functions, stability and adjustable returnability functions, capturing a driver""s intent, and adjustable steering feel functions. On the other hand, the resultant control system structure and control strategy must satisfy closed-loop feedback control system requirements and specifications, such as maintaining stability in the face of uncertainties, rejecting disturbance performance, and quick time response performance.
It is an object of the present invention to control a steer-by-wire steering system for a vehicle. It is another object of the present invention to provide the same functions and feel for an operator of a vehicle having a steer-by-wire system as the functions and feel in a conventional steering system.
It is a further object of the present invention to provide an actuator-based steer-by-wire control system, especially a standard control system structure. It is still a further object of the present invention to provide a steering control system having a steering wheel feedback control subsystem and a road wheel feedback control subsystem.
In carrying out the above objects and other objects and features of the present invention, a steer-by-wire control system is provided that satisfies vehicle steering requirements and the stability and performance requirements. The present invention has a steering wheel control subsystem and a road wheel control subsystem. The subsystems communicate with each other through a subsystem interface.
The steering wheel control subsystem provides the steering feel for the vehicle operator, an angle reference input signal to the road wheel control subsystem, and an active return function with adjustable rates for the steering wheel. The steering wheel control subsystem consists of an inner torque feedback loop, a rate feedback loop and a position feedback loop. The required steering torque that an operator feels is produced by the inner torque loop, the steering wheel rate feedback loop and the steering wheel position feedback loop with programmable steering effort reference curves.
The steering effort reference curves are related to the steering wheel angle, vehicle speed and the torque from the road wheel subsystem. Therefore, in order to reflect the real road condition changes, the road wheel torque signal is used as the feedback signal to form the inner torque feedback loop. A control signal of the road wheel feedback control subsystem can be used as such a torque signal.
In addition to providing the steering feel, the inner torque feedback loop is also used to determine the steering wheel position by working with the other control loops when the operator releases the steering wheel. The position and rate feedback loops also provide an active control function for the variable rate steering wheel return function.
The steering wheel control subsystem provides adjustable steering feel for the operator by controlling the steering torque change in the closed-loop system. It also provides an active, adjustable rate steering wheel return function and the steering wheel positioning function. The steering angle output signal of the steering control subsystem is connected to the road wheel control subsystem as a reference input signal.
The road wheel control subsystem consists of a position feedback control loop and a rate feedback control loop. It is designed as a servo control system so that the road wheel angle tracks the reference input of the steering wheel control subsystem with consideration of the adjustable steering ratio. In order to keep the tracking error in the required minimum range, a gain scheduling control strategy is used to compensate the vehicle dynamics change that occurs with vehicle speed.
The steering wheel control subsystem and road wheel control subsystem are integrated to a steer-by-wire control system by considering the stability and performance of the integrated closed-loop feedback control system. It is a robust control system that can overcome uncertainties and reject disturbances coming from variations in road wheel load, vehicle dynamics, and steering wheel and road wheel systems.
With a steer-by-wire control system, when the vehicle""s operator holds or turns the steering wheel, a suitable operator-adjustable steering feel is produced. At the same time, a road wheel angle reference input is given to the actuator-based road wheel feedback control subsystem. The road wheel angle will automatically track the steering wheel angle. When the operator releases the steering wheel, the road wheel angle and torque will be provided as the reference input signal to the steering wheel control subsystem to change the return rate and determine the steering wheel position. In this case, the steering wheel will return to the center or the desired angle by tracking the road wheel angle with the actively desired return rates.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.